Paramecium Aurelia and Paramecium Caudatum 7 



than the one which would be classed as P, aurelia, while the latter, 

 in turn, measured 90 /* less, on the average, than wild P. caudatum. 

 It is quite apparent, therefore, that size cannot be taken as a diag- 

 nostic character in the present case; and this is, after all, only the 

 application of a well-known principle in zoological taxonomy. 



The pointed condition of the posterior end, also, in P. caudatum 

 is likewise transitory, and may or may not be present in forms which 

 agree in all other characters. Pointed Paramecium, if isolated, 

 and the descendants watched for four or five generations, as I have 

 done, will lose this characteristic and will become rounded and 

 blunt at the posterior end (cf. Figs. 3, 4, and 7). 



The sluggishness which Simpson advanced as a specific char- 

 acter of P. aurelia is purely a physiological condition, depending 

 upon the vitality at a given time, and is as much characteristic of 

 P. caudatum as of P. aurelia. 



The breaking-up of the macronucleus at an earlier period in 

 conjugation, which Maupas considered a diagnostic feature of 

 P. aurelia, may also be due to physiological conditions. I cannot 

 write definitely on this point, as I have had no experience with con- 

 jugating forms of P. aurelia. By itself it would not constitute a 

 diagnostic characteristic of sufficient value to determine a species. 

 So, too, the other characteristic feature of conjugating forms, named 

 by Maupas, the number of corpuscles into which the fertilization 

 micronucleus divides, would be dependent upon the number of 

 micronuclei present, and would amount to the same thing in either 

 case, if each of the two micronuclei of P. aurelia forms four micro- 

 nuclei, as Maupas describes. The experience which I have de- 

 scribed above of the presence of two micronuclei after conjugation 

 in forms which had only one before, indicates that eight corpuscles 

 characteristic of P. caudatum were also formed here, but were 

 resolved into a binucleate instead of a uninucleate condition. This 

 characteristic, therefore, cannot be termed specific. 



Apart from the purely physiological characteristics which have 

 little or no value in classification, there remains only the one specific 

 feature to justify the attempt to separate P. aurelia and P. caudatum, 

 viz., the presence of two and one micronuclei respectively. My 

 experiments show that this, too, is inadequate, for P. caudatum 



